1. Field of the Invention
The present invention relates to a motor driver capable of detecting motor insulation deterioration.
2. Description of the Related Art
Over many years of use, the insulation of a motor deteriorates depending on the environment in which the motor is used and other conditions. If current leaks due to insulation deterioration, a leakage current circuit breaker operates and abruptly stops the operation of the apparatus using the motor. In this case, it takes long time to find the cause of such abrupt stop, as it is unknown whether the problem derives from the motor or the motor driver, with the result that the interruption of the apparatus or production line using the motor tends to extend for a long period of time.
Motor insulation deterioration is generally detected by detecting the leakage current, as described above. Conventional devices for detecting leakage current, such as leakage current detectors and leakage current protection relays, have the drawback of being unable to detect leakage current until insulation deterioration is in an advanced stage, because the detection threshold is normally approximately 15 mA in small detectors, and approximately 3 mA in the smallest detectors. Another drawback of conventional leakage current detectors is that when an apparatus or production line suddenly stops due to leakage current, the entire system operation of the apparatus or production line has to be stopped to investigate whether the problem derives from the motor itself, the motor driver, or a peripheral device. A further drawback of conventional leakage current detection devices is that they are not useful for predicting insulation deterioration for preventive maintenance of the motor.
A method for detecting an insulation deterioration of a compressor motor for an air conditioner at an early stage so as to prevent the air conditioner from becoming inoperative is disclosed in Japanese Patent Application Laid-open No. 2001-141795. According to this method, high frequency pulses are applied to one transistor constituting a power inverter so as to cause leakage current (including current caused by insulation deterioration) to flow into a motor while the motor is not in operation. This inverter changes the voltage of a DC power supply, produced by rectifying and filtering a three-phases AC power supply, into an arbitrary voltage and frequency for driving the motor. An insulation resistance value is computed from the voltage of the DC power supply inputted to a voltage detector and the motor current detected by a current detector. If the computed insulation resistance value is below a predetermined insulation resistance value, an alarm is issued. Another method for predicting insulation deterioration is also disclosed in the above patent document, which computes the insulation resistance value of a motor by simply bringing the transistor into conduction, instead. of applying high frequency pulses to the transistor.
According to the methods described above in the patent document, although useful for predicting and preventing motor insulation deterioration and for preventive maintenance of the motor, detection of voltages and currents are required. Detectors for this purpose must allow for high current, which becomes a cost-raising factor.
In view of the above drawbacks of conventional motor insulation deterioration detection methods, a less expensive motor driver that eliminates these drawbacks and enables insulation deterioration to be predicted has been developed and a patent has been applied for (Japanese patent application No. 2003-177754).
In the motor driver proposed in this Japanese patent application, DC power supply, produced by rectifying the voltage between a grounded AC power supply and the ground, is applied to a through a detection resistor, and the potential difference across the detection resistor is measured to detect motor insulation deterioration. In the motor driver proposed in this Japanese patent application, the voltage applied for detection of insulation resistance becomes a three-phase half-wave rectified voltage in the case of a neutral grounded AC power supply, while it becomes a three-phase half-wave rectified voltage in which one phase out of three phases lacks in the case of a single-line grounded AC power supply, which leads to generation of pulse-shaped voltage, causing a trouble.
In the case of a neutral grounded AC power supply, the applied voltage becomes a three-phase half-wave rectified voltage and is smoothed by a capacitor into a smooth DC voltage. This smooth DC voltage is applied to a series circuit formed by a detection resistor and an insulation resistance of the motor. If the motor insulation resistance lowers, the potential difference across the detection resistance lowers, with the result that the motor insulation deterioration is detectable. In the case of single-line grounded AC power supply, however, the applied voltage is pulse-shaped and is affected by a parasitic capacitance which exists together with, and in parallel with, the motor insulation resistance.
FIG. 4 illustrates the principle of motor insulation resistance measurement in the motor driver proposed in the above Japanese patent application. Reference numeral 51 denotes a three-phase AC power supply and reference numeral 54 denotes electromagnetic switch contacts. Reference numeral 52 denotes a rectifier circuit in the power supply section of the motor driver, which corresponds to the power supply section 6 (to be described later) in FIG. 1 Reference numeral 53 denotes an equivalent circuit representation of the motor drive amplifier, corresponding to the inverter circuit in the motor drive amplifier 8 in FIG. 1. Symbols D51 and D52 denote diodes in the inverter circuit of the motor drive amplifier. Diode D51 corresponds to diodes D11, D12 and D13 in the motor drive amplifier 8 in FIG. 1, and diode D52 corresponds to diodes D14, D15 and D16 in the motor drive amplifier 8 in FIG. 1. Resistors R51 and R52 are equivalent resistors indicating leakage current in the switching elements in the inverter circuit. Resistor R51 corresponds to switching elements Q11, Q12 and Q13 in FIG. 1, and resistor R52 corresponds to switching elements Q14, Q15 and Q16. Symbol RM denotes an insulation resistance between the motor and ground. Symbol CM denotes a parasitic capacitance between the motor and ground. In the motor driver proposed in this Japanese patent application, motor insulation resistance is measured by connecting one phase coil of the motor to the negative-side output line of the rectifier circuit 52 in the power supply section, through the detection resistor R1.
When a motor driver to which such a motor insulation resistance measurement device is attached is connected to the single-line grounded three-phase AC power supply 51 and the electromagnetic switch contacts 54 are closed, a closed circuit is formed in the motor driver by the rectifier circuit 52 in the power supply section, protection resistor R2, detection resistor R1, motor winding (insulation resistance RM between the motor and ground, parasitic capacitance CM between the motor and ground), and grounds G2 and G1, in which three-phase half-wave rectification is performed.
Because the AC power supply 51 has one line grounded, however, a voltage for one phase is removed from among those for three phases and the pulse-shaped waveform P1 shown in FIG. 5, in which the zero potential appears, is applied to the motor. If the motor driver is connected to a neutral-grounded three-phase AC power supply (as in the case disclosed in the above Japanese patent application), a relatively smooth voltage such as one shown by P2 in FIG. 5 is applied to the motor. The voltage waveforms P1 and P2 in FIG. 5 represent peak values of ideal voltage waveforms when a three-phase AC power supply 51 has a phase voltage E of 115 V, rectified with forward voltage drops across diodes in the rectifier circuit and line voltage drops ignored.
If there is no parasitic capacitance CM between the motor and ground, single-line grounded AC power supply and neutral grounded AC power supply produce the same average voltage as far as their phase voltages E are the same, although their peak voltages differ from each other. As a result, motor insulation resistance and its deterioration can be detected at substantially the same level, regardless of how the AC power supply is grounded, by adding a capacitor to the detection resistor R1 to take the average voltage.
In the case of a single-line grounded AC power supply, if there is a parasitic capacitance CM between the motor and ground, the AC power supply produces a pulse-shaped waveform, so that charging takes place only during the width of the pulses and the capacitance CM is alternately charged and discharged. Further, the greater the capacitance CM is, the lower the charging voltage is, resulting in a lower voltage being applied to the motor. If a lower voltage is applied to the motor, the potential difference across the detection resistor increases because the detection resistor is connected in series with the parallel circuit formed by the motor insulation resistance RM and the capacitance CM. Accordingly, to detect a predetermined level of motor insulation resistance, it is necessary to change the value of the detection resistor according to the value of the capacitance CM, or change the reference voltage value to be compared with the voltage detected by the detection resistor. It is difficult, however, to change the values of the detection resistor and reference voltage, because the value of the capacitance CM is unknown.
An insulation tester (megger) for measuring the insulation resistance of the motor to ground uses a smooth DC voltage for detection. Such an insulation tester does not use a pulse-shaped voltage for detection of a motor insulation resistance. Accordingly, if a megger is used to detect an insulation resistance deterioration of the motor in the case of a single-line grounded AC power supply, a conversion table is required for each type of motor.